Apparatus for making precision resistors



NOW 1966 i A. B. CISTOLA 3,282,821

APPARATUS FOR MAKING PRECISION RESISTORS Filed June 13, 1962 FIG. 10

ll" L I2 I2 V///// M. Flaw FIG. 4 "U H11 I F F F iNVENTOR (b) l l l ANTHONY a. CISTOLA (0) UI IL United States Patent 3,282,821 APPARATUS FOR MAKING PRECISION RESISTORS Anthony B. Cistola, Vestal, N.Y., assignor to International Business Machines Corporation, New York, N .Y., a corporation of New York Filed June 13, 1962, Ser. No. 202,236 2 Claims. (Cl. 204-228) The invention pertains broadly to precision electrical resistors, and more particularly to apparatus for treating resistors to bring them within prescribed limits of tolerance.

In the manufacture of certain electrical resistors, a suitable material exhibiting a desired degree of electrical resistivity is applied in a coating on an insulative substrate. Acceptable conventional methods of resistance material application for this purpose are vacuum deposition, silk screening, and the like. However, each of these techniques leaves much to be desired where a high tolerance resistor is required, that is, in the range of one percent (1%) or less.

It is therefore standard practice in the manufacture of high precision electrical resistances to provide a trimming operation subsequent to initial application of the resistance material. For example, according to a usual technique, the resistance material is applied in excessive amount, that is, the resistance value being decidedly less than the desired amount, and, the excess material is removed by grinding or abrading thereby increasing the resistance value to within the desired limits of tolerance. However, due to the necessary exertion of physical pressures on the resistance material in this process there is always danger of damaging the resistor.

It is therefore a primary object of the present invention to provide apparatus for trimming an electrical resistor to Within precise predetermined limits which do not involve the subjection of the resistor to undue physical pressures.

A further object is the provision of such apparatus which includes self-monitoring to prevent over-trimming.

Another object of the invention is the provision of such apparatus cap-able of either increasing or decreasing the electrical value of the resistor, as the case may require.

A still further object of the invention is the provision of apparatus for modifying one or more electrical resistances to a single, or different, predetermined resistance value(s).

Briefly, the invention contemplates immersing the resistor to have its value corrected in an electrolytic solution and by electrolytic techniques varying the electrical value of the resistance. External measuring and control apparatus serve to insure that modification of the resistance is within prescribed settable limits.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.

In the drawings:

FIGURE 1, a and b, illustrates one form of resistor on which the invention can be practiced;

FIGURE 2 is a schematic representation of an electrolytic tank and associated control circuit for practicing the invention;

FIGURE 3 is another embodiment of the invention; and

FIGURE 4, a, b and c, is a graphical representation of electrolytic current in three different operative states of the equipment of FIGURE 3.

Since the theory of operation of the invention is based upon electrolysis, it is important that the resistor to be trimmed, or brought within acceptacle tolerances of resistance value, have exposed surface portions of resistance material which can be acted upon. This means that those types of electrical resistances termed printed or deposited, where the material is applied to a major outer surface of an insulative substrate in a thin coating or film, are especially well suited for application of the method and apparatus of the invention.

Although the utility of the invention is not limited to any particular structural form of resistance, for purposes of illustration a resistance of the general type and geometry shown in FIGURE 1 is used herein. Thus, the resistor 10 is a composite one comprised of an insulative substrate 11 provided with a pair of layers, or coatings, 12 of a material having a suitable electrical resistivity, which material for present purposes will be considered to be carbon. The electrical value of each of the separate coatings 12 is a function of the resistivity of the coating material, the length, L, and the cross-section area, A. Connection wires 13 are connected to the extremity of each coating.

With reference now to FIGURE 2, the resistor 10 (shown in generalized fiorm) is received in uncovered condition within a tank 14. A suitable electrolyte 15 in solution form i provided in suflicient quantity to cover the portions of the resistor 10 carrying the coatings 12. A carbon electrode 16 is also immersed in the same electrolyte in a spaced relation to the resistor 10.

Electrical connections are made to the resistor 10 such that it serves as one leg of a four-legged resistance bridge circuit. Two of the leg resistors, 17 and 18, are of fixed value, and the remaining leg resistor 19 is of the selectively variable variety. An A.C. voltage source 20 is connected from the common point of resistors 18 to 19 and the common point of resistors 17 and 10. The remaining two common points of the resistors 17 and 18 and resistors 10 and 19 serve as an input to an amplifier 21. The output of the amplifier is fed into the actuation coil 22 of a lone-pole relay 23, the other side of which coil is at ground potential of the amplifier. The normallyopen fixed point 24 of the relay is connected to the common point of resistor 10 and 19. The movable strap 25 is tied to the positive terminal of a D.C. voltage source 26, the negative terminal of which is electrically related to the electrode 16.

The values of the fixed resistors 17, 18 and the variable resistor 19 are such that the bridge circuit will be completely balanced (that is, zero sign-a1 voltage to the input of the amplifier 21) when the resistor 10 is precisely at its required resistance value. This, of course, will have to be predetermined for each particular desired value of resistance that is to be trimmed. That is, one set of resistors 1719 interrelated as shown in FIGURE 1 will only be satisfactory for a given small range of resistance value for the resistor 10, the extent of this range being determined by the scope of resistance value provided by the variable resistor 19.

In operation, assume first of all that the resistor 10 has a lower resistance value than is required, that is, it is necessary to increase its electrical value to bring it within the required range. -In this case, the resistor 19 is set to an appropriate value for balancing the bridge circuit when the resistor 10 is of proper value. Accordingly, the bridge circuit at this time is out of balance and an A.C. signal is present at the input to the amplifier 21 providing an output voltage to energize coil 22 closing the normally-open point 24. A closed series path now exists from the D.C. etching voltage 26 across the resistor 10 and the electrode 16 via the electrolytic bath 15. This voltage is of such polarity and magnitude as to immediately initiate etching of the exposed surface portions of the coating, or coatings, 12 serving to diminish the crosssectional area A thereby increasing the measured electrical value of the resistance. Simultaneously during etching, the DC. current drawn through the resistor is reflected as a level change at the input to the amplifier of such polarity as to drive the amplifier in the direction of cut-off. With the resistor 10 of such value as to maintain the bridge unbalanced, the resistor 10 is periodically etched wherein the operating cycle is one of etching, cutofl? of the amplifier, no-etching, and this operation is repeated until the resistor 10 increases in value sufficiently to balance the bridge circuit. When balance is achieved, the signal presented to the input of the amplifier 21 is brought to zero with the result that the coil 22 of the relay 23 is released allowing the relay strap 25 to return to the open position terminating etching of the resistor 10, which is now within the predetermined limits of tolerance.

This feature of etching in short bursts, or cycles, is basically important to the high accuracy achieved by the invention. If, on the other hand, etching current is applied in ;a continuous manner the pH of the bath is changed significantly making the resistor 10 appear to have a correspondingly different value, and any correction applied to the resistor would include this seeming difference of resistance as an error.

Another aspect of the embodiment described above is the capability for simultaneously treating more than one resistor, where the electrical value of each resistor can be the same, or different, from the others. Also, these different resistors to be trimmed can be distinct, separate entities, or, if desired, can be mounted on a common base. This feature is illustrated in FIGURE 2 where the additional resistors 10' and 10" are shown in dashed-line form. In order to accommodate the additional resistors it is necessary that separate electrical circuits be provided for each of the resistors to be trimmed.

Exemplary of one set of satisfactory components and parameters for the embodiment of FIGURE 2 are the following:

Resistor 10 4000 ohms.

Resistors 17, 18, 19.. 4000 ohms. A.C. source l6 volts.

Amplifier 21 D.C. amplifier capable of handling A.C. with a closed loop gain of approximately 10,-

000. Relay 23 8,000-10,000 ohms plate relay. D.C. source 26 Variable tapped battery sufficient to drive the amplifier 21 to beyond cut-elf. Electrolytic bath 15 10% solution by weight of potassium hydroxide buffered to provide a more stable pH.

In cases where it may be considered infeasible to manufacture resistors that initially have electrical values less than that finally desired, the further embodiment of the invention illustrated in FIGURE 3 has particular utility in that, as the case may require, it can either etch away resistance material to increase the electrical value, or it can provide a plating effect over the resistance of a high conductivity material effecting reduction of the total electrical value of the resistor.

The primary differences between the embodiments of FIGURES 2 and 3 exist in the external circuitry. Accordingly as before, a tank 27 is provided which contains a suitable liquid electrolyte 28 for immersibly receiving a resistor 10 to be trimmed and an electrode 29 of a good conducting metal such as copper. The external circuit can now be considered as consisting of two main func tional parts: a pulsing circuit indicated generally at 30 and a plating and etching circuit shown at 31.

As to the pulsing circuit, it consists of a series arrangement of the resistor 10, a variable resistor 32, a capacitance 33, and a DC. voltage source 34. A normallyclosed point 35 of the relay 36 is electrically arranged between the voltage source 34 and the capacitance 33 providing a switching control of the above series circuit. Electrical energization power for the relay coil 37 of the relay 36 is taken across the capacitance 33.

A pair of normally-open points 38 of the relay are shorted together upon energization of the coil 37 to connect power from a DO source 39 to the coil 40 of a plating and etching relay 41. A normally-closed point 42 of the relay 41 is connected to the positive terminal of a center-tapped D.C. supply 43, and a normally-open point 44 of this relay is connected to the negative side of the same power source. The movable strap 45 serves to feed electrical current from the source 43 to the electrode 29 of a polarity depending on the condition of the relay 41. The center-tap connection is made from the source 43 via line 46 to the common point of the resistors 10 and 32.

As the the general principles of operation, the movable strap 45 of the relay 41 serves to place the electrode 29 at either a positive potential (relay is deenergized), or at a negative potential (relay energized) with respect to the resistor 10. As in the first embodiment, if the resistor 10 is positive with respect to the electrode, etching of the resistor will take place resulting in an increase of its electrical value. On the other hand, if the resistor is held at a potential that is negative with respect to the electrode, then in a way well-known in the electrolysis art, plating of the resistor by the highly conductive electrode material is obtained lowering its electrical resistance value. Thus it is seen that dependent upon the condition of the relay 41 the resistor 10 is either etched or plated.

More particularly with respect to the pulsing circuit, the resistor 10, variable resistance 32 and the capacitance 33 cooperatively provide a timed recurrent voltage for picking or energizing the coil 37 of the relay 36. The time constant here is such that a pulse-like voltage symmetrical in time is obtained across the coil 37 when the value of the resistor 10 is within the predetermined limits of tolerance, and, as will be more specifically set forth below, when the resistor is beyond these limits the ratio of energization time to deenergization of the relay coil is correspondingly varied.

It will be assumed for illustration that the resistor 10 is initially of an electrical value less than the tolerance range, and therefore in order to bring it within limits it is necessary to increase its value. In this case, the actuating voltage seen across the capacitance 33, and thus the voltage presented to the relay coil 37, is of such relative periodicity that the relay will be actuated a relatively short time as compared to the time it is deenergized. The points 38 are accordingly closed a comparatively short time, and therefore the plating-etching voltage readings taken across the circuit at V are as illustrated in FIGURE 4a, that is, relatively long periods in which the electrode 29 is positive with respect to the time in which it is negative. Thus, in this case the resistor is etched for substantially longer periods than it is plated, the cumulative effect being an increase in the resistance value.

In the other case Where the resistor 10 initially has a greater electrical value than that which is finally required, the pulse network causes the coil 37 to be energized for a relatively greater portion of the time than it is deenergized. Consequently, the points 38 are closed periodically for a greater portion of the time placing the electrode 29 at a negative potential relative to the resistance 10, which effects a predominance of plating of the resistor 10. For this condition, the voltage is that shown in FIGURE 4b, namely, a relatively longer negative potential impressed on the electrode 29.

When the resistor 10 is brought within the prescribed limits, from either of the above-described initial conditions, the pulse-like energizing voltage for the coil 37 is symmetrical with respect to time and equal times of energization and deenergization result. Accordingly, the relay 41 is picked and dropped out for equal amounts of time, as the electrolytic voltage graph of FIGURE 40 indicates, so

that plating and etching take place at the same rate with the total change of the resistance being substantially zero.

Here, as well as in the first described embodiment, a plurality of resistors (10, 10") either individual in nature or integrally associated, can be trimmed by this technique to the same or different resistance values by providing individual circuits of appropriate characteristics. Modifications of the apparatus of FIGURE 3 to accommodate different values of resistors can be accomplished if the new resistor to be trimmed is not considerably different from the old by simply resetting the variable resistance 32.

Through the practice of the invention, trimming of electrical resistances to within extremely precise prescribed tolerances is made possible. The technique of trimming described herein does not produce hot spots in the final resistance, as sometimes occurs where trimming is accomplished by grinding or abrading portions of the resistance material. This is true because here the complete surface of the resistance material is either removed or plated in a substantially uniform manner.

Although the description herein has been directed toward the modification of electrical resistors in the form of films or coatings, it is clear that the invention would have equal applicability to other types of resistors only requiring variations in the electric current, type and strength of the electrolytic solution, as well as perhaps the substitution or" a diflerent electrodic material for the electrode 29.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit of the invention.

What is claimed is:

1. A system for modifying the resistance value of a resistor, said system comprising a container adapted to contain an electrolyte therein,

means for mounting said resistor in said electrolyte, an electrode and means for mounting said electrode in said electrolyte,

a source of D.C. voltage having positive and negative terminal means associated therewith, and

a control circuit comprising switching means having a first position for connecting said D.C. voltage source in series coupling relationship with said resistor and said electrode mounted in said electrolyte at said positive and negative terminal means, respectively, to effectuate etching action between said resistor and said electrode in said electrolyte,

said switching means further having a second position for connecting said D.C. voltage source in series coupling relationship with said resistor and said electrode at said negative and positive terminal means, respectively, to eflectuate electroplating action between said resistor and said electrode in said electrolyte,

said resistor, and

means responsive to the resistance value of said resistor in said electrolyte to provide a control signal for periodically actuating said switching means, said control signal periodically placing said switching means in its respective first and second positions for equal plating and etching time periods when said resistor 5 has a predetermined resistance value, said control signal periodically placing said switching means in its respective first and second positions for a plating time period longer than the etching time period whenever said resistor has a resistance value higher than said predetermined value, and said control signal periodically placing said switching means in its respective first and second positions for an etching time period longer than the plating time period whenever said resistor has a resistance value lower than said predetermined value, and the periodic actuation of said switching means by said control signal providing periodic reversing of the current in said resistor, said current being derived from said DC. voltage source.

sive means comprises:

an RC network having a condenser and resistance means as the RC elements thereof, said resistance means including said resistor, a second voltage source,

said condenser, and

output means coupled across said condenser to provide said control signal for the actuating of said firstmentioned switching means, said control signal further actuating said second switching means.

References Cited by the Examiner UNITED STATES PATENTS FOREIGN PATENTS 5/ 1958 Canada.

1911 Great Britain. 5/ 195 8 Great Britain.

Triman.

JOHN H. MACK, Primary Examiner.

JOHN R. SPECK, Examiner.

G. KAPLAN, A. B. CURTIS, W. VAN SISE,

Assistant Examiners.

2. A system according to claim 1 wherein said respon Harshaw 204-32 Wallace 204228 Jernstedt 204-228 X Ferrand 204228 Armstrong 204228 X Schnable 20432 Eisler 29-155.62

Hoelzle 156-18 X Pensak 204--228 Mieux 15617 Russell 204212 

1. A SYSTEM FOR MODIFYING THE RESISTANCE VALUE OF A RESISTOR, SAID SYSTEM COMPRISING A CONTAINER ADAPTED TO CONTAIN AN ELECTROYLTE THEREIN, MEANS FOR MOUNTING SAID RESISTOR IN SAID ELECTROLYTE, AN ELECTRODE AND MEANS FOR MOUNTING SAID ELECTRODE IN SAID ELECTROLYTE, A SOURCE OF D.C. VOLTAGE HAVING POSITIVE AND NEGATIVE TERMINAL MEANS ASSOCIATED THEREWITH, AND A CONTROL CIRCIT COMPRISING SWITCHING MEANS HAVIING A FIRST POSITION FOR CONNECTING SAID D.C. VOLTAGE SOURCE IN SERIES COUPLING RELATIONSHIP WITH SAID RESISTOR AND SAID ELECTRODE MOUNTED IN SAID ELECTROLYTE AT SAID POSTIVE AND NEGATIVE TERMINAL MEANS, RESPECTIVELY, TO EFFECTUATE ETCHING ACTION BETWEEN SAOD RESISTOR AND SAID ELECTRODE IN SAID ELECTROLYTE, SAID SWITCHING MEANS FURTHER HAVING A SECOND POSITION FOR CONNECTING SAID D.C. VOLTAGE SOURCE IN SERIES COUPLING RELATIONSHIP WITH SAID RESISTOR AND SAID ELECTRODE AT SAID NEGATIVE AND POSITIVE TERMIINAL MEANS, RESPECTIVELY, TO EFFECTUATE ELECTROPLATING ACTION BETWEEN SAID RESISTOR AND SAID ELECTRODE IN SAID ELECTROLYTE, SAID RESISTOR, AND 